The overall process of hemofiltration which is carried out at the same time as preparation of a replacement fluid is shown, for example, in European Patent Application Nos. EP 0 042 939 and EP 0 087 171, respectively.
Plasmapheresis differs from hemofiltration primarily in that it requires a slightly more permeable membrane material so that even larger molecules can be filtered out therewith.
Examples of suitable hemofiltration membranes are described in European Patent Application No. EP 0 046 816. Similarly, suitable membranes for plasmapheresis are described in European Patent Application No. EP 0 044 958. It will be clear, however, to those of ordinary skill in this art, that other membranes can also be used in conjunction with the realization of the present invention.
When in the past it has been necessary to measure the amounts of filtrate withdrawn in conjunction with processes such as hemofiltration or plasmapheresis, it has been found to be quite difficult to carry out exact measurements due to variations in the pressure conditions and consequent varying flow conditions therein. In particular, specific problems were encountered with peristaltic pumps normally used in such processes. These types of pumps act on the outside of flexible blood tubes in a pulsating manner.
The above-mentioned problem has been largely solved with the help of a system which is described in more detail in European Patent Application No. 85.106708.2. This was achieved with the help of a device for equalizing the pressure in the filtrate flow which passes through the measuring device. In this way, the certainty of the measurement was improved substantially, though not completely satisfactorily. It is suspected that the residual uncertainty is due to the microbubbles formed during the pumping which pass through the pressure equalizing device that is designed to separate air from the filtrate.